There are algorithms that calibrate a low precision clock with respect to a high precision clock, also referred to as a master clock. This allows a low precision clock to produce timing nearly as precisely as that of the master clock. These techniques have one thing in common—to calibrate the low precision clock periodically with respect to a master clock.
In battery-operated devices such as wireless transmit/receive units (WTRUs) and other mobile devices, it is very important to limit power consumption to extend battery life. Algorithms and hardware in the WTRU should be designed to minimize power consumption. Battery life can also be extended by reducing power consumption during periods of inactivity in which certain functions can be turned-off or operated in some form of reduced-power mode. The UMTS is configured such that a WTRU can operate with reduced functions during periods of inactivity. The WTRU need only occasionally perform certain functions to maintain synchronization and communications with its associated base station while a call or other dedicated connection is not in progress provides the periods of inactivity which can allow the WTRU to minimize its power consumption. This is achieved by the WTRU operating using discontinuous reception (DRX), wherein the WTRU periodically cycles between “sleep” and “wake” periods. During sleep periods, unneeded power-exhausting processes and hardware are turned off. During wake periods, these processes and hardware, needed to maintain synchronization and communications with the associated base station, are momentarily turned back on.
Most handheld WTRUs today include a low precision real time clock (RTC) in addition to the high precision master clock. The master clock is typically implemented using a temperature controlled crystal oscillator (TCXO). The RTC typically consumes much less energy than does a TCXO, making it desirable to use the RTC instead of the TCXO to provide timing functions during DRX. There are, however, four problems with using an RTC for timing during DRX. Firstly, the RTC typically operates at a greatly reduced speed as compared to TCXO (e.g., 32,768 KHz vs. 76.8 MHz). Second, the frequency accuracy of the RTC may be very low compared to that of a TCXO. Third, the frequency drift of the RTC due to different environmental reasons, such as temperature changes, may be greater than that of a TCXO. Fourth, the RTC typically operates asynchronously to the TCXO. For these reasons, a typical RTC is, alone, inadequate to supply timing functions during DRX.